OY, the argument is that you do get a skill test. Every single year to maintain your instrument rating, unlike the USA system. Under UK/JAR & also Oz, an instrument rating is time limited. It last a year & then it's gone - unless you do a test to renew it, even AFTER the issue of an ATPL. A current IR is required for the issue of the ATPL. That means you must have done an IR test within the last year.
The USA system doesn't require that. One Commercial or IR test & it's pretty well valid forever. You could front up 10 or 15 years later having done no tests (other than a BFR), hence the need for an ATP issue test. Don't forget that the ATP checkride is basically an IR test. Note that thereafter instrument privileges are held with the ATP, so again, no further testing of skill may ever be taken but the pilot still maintains instrument privileges.
Each system has its merits and its disadvantages. End result is that both systems achieve similar levels of safety although I think the UK/JAR system is bureacratically expensive nightmare compared to the US or Oz.